docs: def gap_sizes(self):
          """Return gap sizes per chromosome.
          
          Returns
          -------
          gap_sizes : dict
              a dictionary with chromosomes as key and the total number of
              Ns as values
          """ 
          if not self._gap_sizes:
              gap_file = self.props["gaps"]["gaps"]
              self._gap_sizes = {}
              with open(gap_file) as f:
                  for line in f:
                      chrom, start, end = line.strip().split("\t")
                      start, end = int(start), int(end)
                      self._gap_sizes[chrom] = self._gap_sizes.get(chrom, 0) + end - start
          return self._gap_sizes